How to Create Dust Floating in the Air?
Mix an volumetric environment with a particle system:


density of volume scattering will determine the amount of microscopic dust particles that create light rays
particle system will add visible dust


Points to keep in mind:


create the light super-bright and scatter density tiny - that way the small particles get enough light.
give the particles translucent shader or a bit mixed-in emission if they are too dark
the particles need to appear small on camera so they blend with the light-shafts (smaller the better). Particles should be also random in size.


A closeup of such out-of-focus dust:



the black border is there to fight the white page background

Settings (depends on scene size, light distance and camera):


scatter density: 0.003
used light: Point, strength: 5000
particles: 1M
particle size: 0.015, random size: 100%
icosphere scale: 0.1